A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for various applications. In particular, individual fuel cells can be stacked together in series to form a fuel cell stack capable of supplying a quantity of electricity sufficient to power an electric vehicle. The fuel cell stack has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
Fuel cells are electrochemical devices which combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode, an anode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrolyte-assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between diffusion media or diffusion layers (DM) that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. In certain designs, the cathode and anode are also formed on the DM and sandwich the electrolyte membrane. The DM serve as current collectors for the anode and cathode as well as provide mechanical support for the MEA. The DM and MEA are pressed between a pair of electronically conductive bipolar plates which also serve as current collectors for collecting the current from the electrochemical fuel cell reaction.
The bipolar plate typically includes two thin, facing metal unipolar plates. One of the metal unipolar plates defines a flow path on one outer surface thereof for delivery of hydrogen reactant to the anode of the MEA. An outer surface of the other unipolar plate defines a flow path for the oxidant reactant for delivery to the cathode side of the MEA. When the unipolar plates are joined, the joined surfaces define a path for a coolant fluid to flow therethrough. The unipolar plates are typically produced from a formable metal that provides suitable strength, electrical conductivity, and corrosion resistance, such as 316L alloy stainless steel, for example.
The fuel cell stack is generally compressed to hold the various components thereof together in operation. To militate against undesirable leakage of reactants and other fluids from the fuel cell stack, a seal is often employed. The seal may be provided by a gasket, for example. The seal may also be disposed along a peripheral edge of the pairs of plates.
Known seals have been formed from an elastomeric material. The seals formed by the elastomeric materials have performed adequately for prototyping. However, the cost of the elastomeric materials makes the use thereof undesirable for full scale manufacturing. Another known seal results from a line-contact between adjacent metal bead seals formed on the plates. However, an undesirable “shingling” or offsetting of the bead seals during compression of the fuel cell stack may result. A lateral movement of the fuel cell stack components during an operation of the fuel cell stack may also result in an undesirable leakage of fluids therefrom.
There is a continuing need for a robust bead seal between the plates of the fuel cell stack that militates against a leakage of fluids from the fuel cell stack and that is cost-effective in manufacturing.